The present invention relates to a process for the inspection of transparent sheets passing by continuously or in separate sections, in order to detect flaws, even minimal flaws, which the sheet might include. Further, the process does not detect foreign bodies or soil that may be present on the sheet surface which bodies or soil to not affect the planar nature of the sheet and may be eliminated by a simple washing. The present invention also relates to an apparatus for applying the process.
Processes are known, together with devices for their application, that are capable under certain conditions of detecting flaws present on the surface of a flexible film, such as a photographic film.
The object in these known processes is to detect all of the flaws of the support or of an emulsion deposited on the film, together with all of the soil or foreign bodies present on the surface. It is very difficult to eliminate the soil from the film by simple washing since the emulsion cannot tolerate such a washing. In other words, anything that may alter the purity of the surface of the film inspected must be considered a flaw.
The situation is entirely different in the case of the inspection of glass during its manufacture by the process, for example, of flotation on a bath of molten tin, designated the "float glass" process. In this case, only the flaws of the materials itself: bubbles, drops, inclusions, projections of tin, for example, are to be detected, because they affect the flatness and the transparency of the glass. On the other hand, it is unnecessary to detect simple dirt of any nature found deposited on the surface, because simple washing is sufficient to eliminate the dirt. The detection of dirt as a flaw may lead to the rejection of a perfectly usable portion of the glass.
It is therefore desirable to effect in this case a finely selective method of detection, capable of substantially perfect discrimination between flaws of the material and surface soil that may be carried by the material.
Furthermore, the known processes generally use a laser beam made to scan the surface of the material in a cyclic manner, and reflect from an assembly of rotating or oscillating mirrors.
Such processes thus include mechanical elements which are a source of imperfection and make difficult the perfect synchronization between optical scanning by the laser beam and the velocity of the passage of the glass surface particularly when it is necessary to modify the glass surface velocity. Accordingly, such a process may impose the condition of a constant velocity, which represents a restriction that may become a disadvantage.
Finally, the cost of the use of devices for the application of such processes is high, due to the high expense of the source of the laser beam. Also, the laser beam source in a glass manufacturing plant is subject to a high risk of deterioration.
These disadvantages apply among others to a device of the type described in French Patent No. 2,238,930 and to the earlier patents cited in this patent.
Furthermore, most of the known processes require frequent standardization as the result of the variations that may occur either in the intensity of the luminous source or in the reflectivity of the rotating or oscillating scanning mirrors which may rapidly become charged with impurities. Standardization may also be required for even a simple change in the thickness of the glass from one product to another or in the refringence or the tint of the glass. Such standardization is actually necessary practically for each scanning resulting in an increased complexity of the automatic control system and thus in an increase in cost.
Finally, concerning devices which operate by reflection from the sheet of glass to be inspected, it has been found that they are not suitable for the inspection of continuous sheets of glass. In actual fact, the length of the band in this case and the stresses acting upon the sheets are such that the surface is almost constantly exposed to vibrations, which cause aberrations in the reflected beam, if such devices are used.
According to certain other processes, the radiation used consists of infrared rays. Such a process and device are described in French Patent Nos. 1,435,210 and 2,099,921. These arrangements do not permit a selection among flaws of the material and simple soil that adheres to the material surface. Detection of soil may be satisfactory or even necessary for the object of the two patents which relate to the control of the quality of photographic strips but such detection is not satisfactory for the purpose of the present invention, as this would result in the automatic rejection of portions of the glass that are simply soiled but perfectly usable after a simple washing.
It is to be noted further that in each of these patents the source of light is located in the geometric plane which also contains the flaw to be detected, the optical axis of the focussing device and the detector element.
The deviation of the incident luminous rays, either by reflection or by refraction, due to the deformation of the surface of the material produced by a flaw which deforms the material slightly causes in each of these cases a reduction in the quantity of light normally received by the detector element. In other words, the deviation diverts the light rays from the optical axis of the captor upon which the liminous source is located. The image of a flaw is thus always a more or less dark spot appearing on a peripheral bright field. This process is designated the "bright field" projection process hereinafter.
It is seen therefore that any foreign body located on the surface of the glass will be detected in the same manner as a flaw of the material itself.
While nonselective detection of this type is desirable in the case of the inspection of emulsified photographic film, the situation is different in the inspection of glass wherein only material flaws are to be considered, regardless of their nature. These flaws include even partial inclusions of foreign bodies, but exclude any soiling due to the presence of the same foreign bodies, when they are simply positioned on the surface of the glass without deforming the surface since such foreign bodies are capable of elimination by a simple washing process.
It is further noted that the contrast created by a dark image on a bright background is difficult to detect. Accordingly, such an arrangement limits the fineness of such detections.
It is therefore an object of the present invention to attain the result necessary in the inspection of sheets of glass in the course of their manufacture by eliminating the aforementioned disadvantages inherent in each of the known processes when applied to the specific case under investigation. The process desired should be perfectly selective and should allow the elimination of only those portions of the glass being produced which portions exhibit real flaws of the material, regardless of their nature or dimensions, even the smallest flaws, without the indication of superficial soil which does not mark the surface of the glass.
In order to accomplish this object according to the principles of the present invention, the deviation of the light rays caused by the presence of a flaw affecting the surface of the glass and, as indicated hereinabove, diminishing the intensity of the image yielded by the optics of the source of light located on its axis, is utilized here, in contrast to the known processes, to cause an increase in the light with respect to the general image produced by the same source. In this way, it is possible, as may be seen and in contrast to the other processes, to differentiate flaws of the material from simple soil deposited on that surface which soil still causes a reduction in the luminous intensity by simple occultation according to the present invention.
It has been found in fact that any flaw of the material, whether it consists of bubbles, blow holes, drops, or even partial inclusions, causes flatness defects on the glass surface. In other words, a more or less severe deformation of the surface is found at this level, which, however, is always present. The object of the present invention is thus to utilize the deviation of the luminous rays caused by a surface flaw of the glass to render the image even more luminous than the general image of the source of light. This deviation being obtained by the effect of refraction in the mass of glass deformed. This effect is not produced by simple soiling.
In the present process of discriminating against flaws due to dirt, the use of the deviation of light rays by diffraction, commonly used in the so-called "dark field" process, has been eliminated. The "dark field" process consists of darkening the light in the center of the optics, so that no rays are penetrating directly into said optics. The object to be observed is illuminated solely in an oblique manner by intense peripheral rays. The rays are diffracted by the object and are the only ones penetrating into the optics, thereby forming a clear image of the object on a dark background.
It is obvious in this case, that the flaws of the surface will produce a luminous zone on a dark background, which zone because of this fact may be detected more readily than the previously described inverse image, by virtue of its better contrast. However, independently of the fact that such a process requires an intense source of light, it has been found that the images of all opaque foreign bodies, which generally constitute the soil deposited on the surface of the glass, also appear haloed by light on the black background. This detection of foreign bodies causes the actuation of any photosensitive device in the same manner as a surface defect. Consequently, selectivity of the process is eliminated.
For this reason, according to the present invention, it is not diffraction that is applied to the surface defects of the glass, but refraction. The defects are able to produce refraction in the same manner as a prism or a lens, as they may be considered generally to be aspherical, warped lenses of a more or less pronounced configuration, and having a more or less oblique optical axis.
The process that is the object of the present invention thus consists, according to a preferred embodiment, of utilizing the deviation of the incident luminous beam due to the deformation of the surface of the material produced by a flaw, to increase the quantity of light normally received by the captor toward the optical axis from which the deviation is randomly projecting luminous rays that normally are not perceived. Such an increase in light could not be caused in any case by simple dirt. To accomplish this result, a point source of light with a low intensity is used. The light is placed, in contrast to the known processes, outside but in close proximity to the optical plane also containing the optical axis of a focussing device at the end of which is located the photosensitive detection element.
In this manner, a condition that may be called a "mixed field" is created. The "mixed field" may be understood by recalling that the "bright field" is produced when the observer, or the photosensitive element, and the luminous source are on the same optical axis, which also contains the flaw to be observed. The flaw appears as a dark spot on a bright background. Since the flaw has refracted the light outside the optical field, or darkened the light if an opaque foreign body is involved. The result appears in an identical fashion as a dark spot on a bright background, which spot is difficult to observe and nonselective. In the "dark field", which also assumes the alignment on the optical axis of the observer, the source of light and the object is obtained by illuminating the object by an intense peripheral beam which is diffracted toward the optical axis, causing a luminous image of the object to appear on the dark background. This luminous image is not any more selective than the dark spot, even though it has more contrast.
The "mixed field" thus has the effect of rendering the image of a surface flaw of a sheet of glass in the form of a bright image on a dark background, essentially as in the case of the dark field. However, the bright image is no longer an effect of diffraction but rather one of refraction, which has the essential and novel consequence that a selection between flaws of the material and dirt is assured, as the dirt cannot create the refractive effect desired. Accordingly, the "mixed field" is different from that of the dark field. The "mixed field" also has the further advantage of providing a substantial contrast in the detection of flaws which contrast comprises a bright image on a dark background, different from the result of the bright field.
This result is obtained according to the process of the present invention by distancing the low intensity point source of light away from the optical axis of the detection system by a small value but one that is sufficient so that the light captor is at the limits of the image produced in its plane of the source of light by the optical system. With this arrangement, in the absence of a deformation of the surface of the glass, the observer or the photosensitive element substituted for the observer can receive only the limiting fringe of the image of the luminous source. This arrangement assures a generally low level of illumination and is called being at the limits of the "bright field".
It may be understood then that the slightest deviation of the light rays caused by refraction due to an internal or superficial deformation of the glass will cause the rays penetration into the optical system. This refraction produces in the plane of the captor an image with a higher luminous intensity, defined in a perfectly contrasted manner on the dark fringe of the image of the light source which remains unchanged and at the outside of which the luminous image of the flaw appears.
It will also be understood that only flaws of the glass are capable of creating the refraction necessary to form the luminous image. Dirt appears only as a deeper darkening of the mixed field, which makes it possible to distinguish it from the flaws investigated. This differentiation of dirt and flaws is facilitated even more by the fact that the light source is of low intensity, thereby eliminating the possibility of diffraction at the level of the periphery of soil.
The process according to the present invention thus makes it possible to obtain the selectivity desired while providing a fineness of the detection of flaws limited only by the sensitivity of the photosensitive detector elements.
The device for the application of the process of the present invention thus essentially comprises a filiform, i.e., thread-like source of light adapted to cover the entire width of the band to be inspected and placed under the sheet of glass passing above the light source. The source of light comprises one or more fluorescent tubes, installed for example end to end depending on the width of the sheet of glass to be inspected, and placed transversely with respect to the direction of passage of the glass. In order to illuminate its entire width, a narrow, longitudinal slit is provided in the cox containing the sources of light. An optical focussing device is placed over the glass to be inspected. The focussing device is located in relation to the light source and the sheet of glass such that each of them is outside their respective foci, to form a real, clear image of the surface of the glass and of all that it supports in the plane. An assembly of juxtaposed photodiodes is placed for exploring the entirety of the image formed in a continuous manner.
And, as mentioned hereinabove, the source of light is removed from the optical axis of the system by a value just sufficient so that the grid of photodiodes is located on the extreme fringe of the light source image and parallel to the light source. In this manner, in the absence of any deformation of the surface of the glass, the photodiodes are impacted only slightly, in a uniform manner, in keeping with a relatively low threshold, designated the "light threshold". In contrast, upon the appearance of any deformation of the surface of the glass, even when very small, the light rays are sufficiently deviated by refraction so that some of the light rays will traverse the optical system, thereby forming a luminous image which impacts certain of the photodiodes, stressing them at a very precise point until a higher threshold, designated the "flaw threshold" is attained. Reaching the "flaw threshold" causes a signal of known coordinates to be emitted which will be processed by an electronic assembly actuating a system for issuing instructions. The instructions are interpreted to eliminate or to simply mark the part of the glass affected by the defect.
It will be understood further that an impurity that does not deform the surface of the glass and therefore does not constitute a permanent flaw, will produce in such a system an even more substantial weakening of the luminous intensity of the image received by the captor. The image at this point passes under the standard light threshold, so that the signal emitted in this case by any one of the photodiodes will obviously be different from that generated by a flaw of the material.